This invention relates to a process for producing hydrocarbons from a subterranean formation. More specifically, the invention relates to a method of using wet electric heating to facilitate hydrocarbon production, and more particularly, producing hydrocarbons having pre-heated viscosities of about 100 centipoise or greater.
Much of the hydrocarbons produced under primary methods (i.e., non-thermal processes) has a viscosity, ranging from about 0.5 centipoise (xe2x80x9ccpxe2x80x9d) to about 100 cp. Because of this relatively low viscosity, a significant percentage of the oil in place (xe2x80x9cOIPxe2x80x9d) in the subterranean formation can be produced without resorting to thermal processes. Typically the percentage of the OIP that can be produced under primary methods will range from about 3% to about 30%.
However, there are significant deposits having higher viscosity hydrocarbons with pre-heated viscosities in the range from about 100 cp to about 1,000,000 cp or even greater. Typically, for a subterranean formation containing hydrocarbons with a pre-heated viscosity of about 100 cp to about 1,000 cp, roughly 3 to 10% of OIP can be recovered using conventional primary techniques. To produce beyond that percentage, of course, requires one or more processes, including among others, thermal processes (i.e., secondary recovery).
For convenience, hydrocarbons with pre-heated viscosities in the about 100 cp to about 1,000 cp range will be referred to herein as xe2x80x9cheavy oil,xe2x80x9d while hydrocarbons with pre-heated viscosities in the range of greater than about 1,000 cp to about 1,000,000 cp or greater will be referred to herein as xe2x80x9csuper heavy oil.xe2x80x9d One of the more common types of super heavy oil is tar sands, also known as oil sands or bituminous sands.
Tar sand deposits are impregnated with dense, viscous hydrocarbons and are typically a mixture of sand, water and bitumen. Bitumen is a hydrogen-deficient oil that can be upgraded to a commercially desirable hydrogen to carbon ratio by carbon removal (i.e., coking) or hydrogen addition (i.e., hydrocracking). The sand component in a tar sands deposit is primarily quartz, which is typically about 80% to 85% by weight (xe2x80x9cwtxe2x80x9d) of the deposit, while the remainder is bitumen and water, which comprises about 15 wt % to 20 wt % of the tar sands.
Worldwide tar sand deposits can provide an enormous resource of hydrocarbon reserves. In September, 1982, during the Proceedings of the Second International Conference on Heavy Crude and Tar Sands (Caracas, Venezuela), R. F. Meyer and P. A. Fulton estimated the total bitumen in place globally as 4.07xc3x971012 barrels (xe2x80x9cbblxe2x80x9d) (about 4 trillion bbl). Of this total bitumen in place, they estimated about 2.4xc3x971012 bbl in seven deposits in Alberta, Canada, about 1xc3x971012 bbl in four deposits in Venezuela, about 5.6xc3x971011 bbl (0.56 trillion bbl) in Russia and about 3.4xc3x9710110 (0.034 trillion bbl) in 53 deposits in the United States.
Of course, because of bitumen""s high viscosity and the intimate mixture bitumen forms with sand and connate water, tar sand deposits and other super heavy oil deposits cannot be exploited using primary oil recovery techniques. Therefore, the super heavy oil (e.g., bitumen) has often been mined, presuming the deposit is at a sufficiently shallow depth, or otherwise produced using a non-mining, but enhanced recovery, process.
Non-mining processes that may be used include thermal and non-thermal processes. Non-thermal processes can include cold production (i.e., sand production) and solvent injection, while thermal processes can include in-situ combustion or a hot aqueous fluid injection and displacement or drive process using hot water, steam or a steam/solvent mixture. But typically a hot aqueous fluid, such as hot water or steam, is used to reduce oil viscosity and displace the oil. For example, one common heavy oil or super heavy oil recovery technique involves steam injection, followed by a steam xe2x80x9csoakingxe2x80x9d phase and subsequent recovery of the reduced viscosity oil, also known as huff-n-puff or cyclic steam stimulation (xe2x80x9cCSSxe2x80x9d). Huff-n-puff or CSS can also be combined with an electric heating process to provide additional heat and viscosity reduction.
For example, in U.S. Pat. No. 3,946,809 (Mar. 30, 1976), Hagedorn suggests that CSS should be followed by electric heating so that brine can be injected into the region where the oil was displaced under the CSS process. Specifically, Hagedorn""s proposed process involves four steps: (1) CSS, which is terminated when there is interconnection of CSS heated zones between wells; (2) producing oil and water; (3) injecting high conductivity fluid into CSS heated zones; and (4) completing wells as electrodes and allowing current to flow between wells to increase the temperature of oil not heated in CSS. And more specifically, Hagedorn suggests that the volume of high conductivity fluid should be sufficient to displace substantially all water condensed from steam from the CSS heated zones. But Hagedorn warns that xe2x80x9cthe volume should not be so great, however, as to displace substantial amounts of high-electrical-resistivity connate water from the unheated portion of the reservoirxe2x80x9d (col. 6:1-4).
As discussed in more detail below, it is well understood by those skilled in the art of thermal oil recovery processes that when steam is injected into a formation, it will rise forming a conical bowl steam zone around a vertical well. See for example, Boberg, T. C. Thermal Methods of Oil Recovery John Wiley and Sons, 411 pgs.; pg. 166; 1988 and Butler, R. M. Thermal Recovery of Oil and Bitumen Prentice Hall, 528 pgs., pg. 258-259; 1991.
So, Hagedorn suggested either prohibiting or restricting the amount of electrolytic or high conductivity fluid (e.g., brine solution) introduced into the unheated portion of a reservoir, where oil was still substantially in place, was important in practicing an electric heating process. This was understandable since it was generally believed by Hagedorn and others skilled in the art then, and up to now, that increasing the electrode zone""s effective radius was, alone, the critical factor to effectively electrically heat a formation, while ignoring electrode zone spacing, geometric shape and spatial orientation effects. However, surprisingly and unexpectedly, the inventors have discovered that, by properly accounting for electrode zone spacing, geometric shape and/or spatial orientation effects in substantial accordance with the detailed description provided below, a target region in a formation heating will be more diffuse than in a conventional electric heating process, like Hagedorn""s for example, that fails to properly account for spacing between electrode zones, geometry effects (e.g., electrode zone surface area and shape) and/or electrode zone spatial orientation.
For example, in a CSS configuration, such as Hagedorn used, it is important to ensure that an electrolytic or high electric conductivity fluid is in place in both the unheated, as well as any previously heated portions of the reservoir, contrary to what Hagedorn, in fact, taught. Put another way, beyond the electrode zone""s size, it is also important to ensure that the spacing, geometric shape and/or spatial orientation of the electrode zone formed with the injected electrolytic fluid has a suitable combination of surface area and shape for eliminating or reducing, among other things, unwanted xe2x80x9cedgexe2x80x9d effects. xe2x80x9cEdgexe2x80x9d effects lead to undesired small volume xe2x80x9chot spotsxe2x80x9d (i.e., more intensely heated regions), rather than relatively more diffuse heating between electrode zones, like that generated with the inventive WEH process more fully described below.
Consequently, while Hagedorn and other proponents of electric heating processes in oil formations have focused primarily on the electrode zone""s size, they have, in the meantime, overlooked and/or incorrectly assessed the effects that electrode zone spacing, geometric shape and/or spatial orientation would have on significantly improving electric heating rate and distribution. Also, another factor that has been overlooked and/or incorrectly assessed is the relative magnitude of the effective electrode zone diameter and the distance between wells.
More specifically, while a CSS steam process can produce an elliptical cross-sectional area at the top of the CSS steam zone, as illustrated in Hagedorn""s FIG. 2, this elliptical cross-sectional area does not extend along the entire, much less a substantial portion of, the wellbore""s length. Instead, the CSS steam zone is a conical bowl-like shape (vs. an elliptical cylinder shape), narrowing down to substantially the diameter of the wellbore itself at the bottom of the injection zone, where the electrode zone diameter is significantly smaller than the distance between wells, compared to the top of the conical bowl. Therefore, when high conductivity fluid is injected into the CSS steam zone, in the manner Hagedorn describes, so as not to displace connate water outside the CSS zone, the injected fluid will form a conical bowl-shaped electrode zone around the well. Then, when a current flows between the electrodes, a point source is created between facing edges of the top elliptical surface of the bowls. But little to no heating occurs between the electrode zones below the top surfaces of the bowls.
Moreover, hot spots at the point sources can overheat the connate water around the point sources. And when the connate water is overheated, water vaporizes to steam, thereby potentially disrupting electrical connectivity between the electrodes, depending on the proximity of the hot spot to the conductor. Thereafter, current flow may be disrupted between the electrode zones, thereby disrupting any further electric heating. Of course, this type of performance is generally unacceptable to the oil and gas industry and illustrates why the industry has remained reluctant to deploy the conventional electric heating processes known to those skilled in the art up to now.
Hagedorn""s disclosure, therefore, illustrates how those skilled in the art of thermal recovery processes, more particularly, electric heating processes, have understood the potential benefit of using an electrolytic fluid to enhance an electric heating process. But likewise, Hagedorn""s disclosure, among others, also illustrates how those skilled in the art have failed to appreciate and understand the importance of using a suitable combination of electrode zone surface area, shape and spatial orientation to generate significantly improved electric heating rates and distribution between electrode zones vs. the heating rates and distribution generated by conventional electric heating methods, in which the electrode zone spacing, geometric shape and/or spatial orientation have been overlooked and/or incorrectly assessed.
In addition to CSS, steam assisted gravity drainage (xe2x80x9cSAGDxe2x80x9d) techniques, such as those disclosed by Butler in U.S. Pat. No. 4,344,485 and Edmunds in CA U.S. Pat. No. 1,304,287, each incorporated herein by reference, can also be used to recover heavy oil and super heavy oil from subterranean formations. These non-drive, non-displacement techniques rely primarily on producing a steam chamber covering a large surface area in the formation near the region where heavy oil is located, while also relying on the thermal conduction effect and, to some degree, convective heat transfer at the steam front, to ultimately heat the nearby heavy oil, thereby lowering its viscosity and increasing its flowability accordingly. In turn, the oil can flow simply under the influence of gravity, rather than by a displacement or drive process, to a second well, which is normally a horizontal production well.
During the SAGD initialization phase little to no oil is produced, but with continued steam injection a steam chamber is produced and fluid communication with a second well is established. In accordance with Butler""s disclosure in U.S. Pat. No. 4,344,485, for his disclosed SAGD process he states that xe2x80x9cto be practical, it is necessary to develop steam chambers having very large surface areas relatively quickly.xe2x80x9d (see col. 8:27-30). To achieve this result, Butler suggests developing a vertical fracture between an injection well and production well and injecting steam into the fracture to create a steam chamber with a narrow width but considerable vertical and horizontal dimensions with respect to the vertical fracture. Accordingly, thermal communication between the injection and production wells is then established as the region surrounding the fracture becomes saturated with steam. In accordance with the SAGD process Edmunds discloses in CA 1,304,287, the formation is not fractured, but rather the initialization phase first requires fluid communication between the production and injection wells to establish thermal communication for creating a steam chamber covering a relatively large surface area of the formation. Commonly this is achieved by circulating steam independently in each well. Consequently, this can make the process time consuming, while also requiring significant energy to initialize the process.
Unfortunately though, the initialization phase for a SAGD process, whether by either of these disclosures, relies mainly on thermal conduction through the formation, while convective heat transfer, if any, becomes less a contributing factor in enhancing the rate the steam chamber is developed as the viscosity of the oil in place increases. So, SAGD initialization can be time consuming and costly when using steam exclusively as the heating source, despite fracturing techniques like those suggested by Butler in U.S. Pat. No. 4,344,485.
Similarly, the Vapex process, which is closely related to the SAGD process, uses propane alone (Dry Vapex) or a propane/steam mixture (Wet Vapex) to create a communication path between an injection well and production well. In the Wet Vapex process there are two fluid containing chambers. The first chamber is a SAGD-like steam chamber, but which contains both steam and a hydrocarbon vapor near its condensation point (i.e., wet hydrocarbon vapor, hence xe2x80x9cWet Vapexxe2x80x9d) and a second larger chamber containing propane (C3), primarily in a gaseous state. The Wet Vapex process is described more fully in the SPE paper xe2x80x9cIn-Situ Upgrading of Heavy Oils and Bitumen by Propane Deasphalting: The Vapex Processxe2x80x9d (SPE 25452 I. J. Mokrys and R. M. Butler, presented Mar. 21-23, 1993 at the Production Operations Symposium, Oklahoma City, Okla.), which suggests, for example, that propane is injected with steam to produce both a steam/C3 chamber and a lower temperature C3 chamber. The steam chamber in the vicinity of the injection and production wells strips propane from the oil, while the stripped C3 is recycled internally into the lower temperature C3 chamber that spreads laterally into the formation where it dilutes, upgrades and extracts the oil. But before producing the steam/C3 and C3 chambers, the authors suggest initializing a Wet Vapex process with steam alone to create a communication path between the injection and production wells. Again, however, in field use, this steam initialization phase is time consuming. Moreover, the conventional steam initialization phase can often adversely affect the economics of the Wet Vapex or any other steam-based process that uses one or more fluid chambers for conductive heating.
In the Dry Vapex process, described in U.S. Pat. No. 5,407,009 (Butler et al., Apr. 18, 1995) and U.S. Pat. No. 5,607,016 (Butler, Mar. 4, 1997), solvent vapor is injected into an aquifer located below the hydrocarbon deposit. Solvent vapor is injected with a less soluble gas, such as natural gas or nitrogen, to mobilize hydrocarbons.
Steam is commonly used as a heat source for establishing fluid communication between wells and/or for thermal recovery processes. However, heating with steam relies on thermal conduction, which can be time-consuming. Accordingly, alternative heat sources have been proposed. One alternative to steam heating is electric heating, which has been proposed for reducing hydrocarbon viscosity. However, the prevailing view in the industry is that, absent special measures to improve uniform formation heating relatively comparable to or better than steam, electric heating is wasteful and uneconomical, and most particularly uneconomical for tar sand deposits. Also, depending on the conversion process used and the operating conditions, converting fossil fuel energy to electric power is only about 30 to 40 percent efficient.
U.S. Pat. No. 4,926,941 by Glandt et al. (May 22, 1990) proposes a process for electric heating of tar sand deposits containing thin, high conductivity layers, which are typically shales that have tar sands alluvially deposited (i.e., by flow of water) within them. Glandt et al. propose that a thin conductive layer, such as a shale, is heated to a temperature sufficient to form an adjacent thin preheated zone, in which the viscosity of the tar is reduced enough to permit steam injection into the thin preheated zone. Electric heating is then discontinued and the deposit is steam flooded. According to Glandt et al., this electric heating generates a uniformly heated plane, such as the shale layer, within the tar sand deposit. However, this technique for electric heating clearly requires a shale layer or similar type of naturally occurring thin conductive heating layer. Consequently, there are formation requirements limiting where this heating technique can be used effectively. Moreover, the requirement for a thin conductive layer makes the process poorly adaptable to non-displacement processes, such as SAGD.
Also, U.S. Pat. No. 4,620,592 by Perkins (Nov. 4, 1986) discloses an electric heating method where a formation with multiple sets of a plurality of spaced apart wells is progressively produced in a preselected direction. A first set of wells is used to both apply electric heat to the formation and inject brine. Then electric heating and brine injection are applied to a second set of wells spaced in a preselected direction from the first set of wells. Thereafter, electric heating in the first set of wells is ceased and hot aqueous fluid injection is commenced. These steps are sequentially moved to co-act with each while traversing the formation and thereby producing the formation in a more energy efficient manner. Again, however, this combined technique of electric heating with a fluid displacement is poorly adaptable to non-displacement processes, such as SAGD.
Moreover, each of the processes discussed above and other electric heating processes for hydrocarbon containing formations has not used the electric heating most efficiently. Also, as indicated by each of the above disclosures, those skilled in the art have routinely relied on using electric heating in combination with a fluid displacement or drive process to provide more uniform electric heating.
Accordingly, there is a need for an improved electric heating process that can effectively. operate without necessarily requiring a displacement or drive process to provide more diffuse electric heating of a formation, particularly a formation containing heavy oil or super heavy oil. Also, there is a need for an electric heating process that provides more diffuse electric heating in a target region between electrodes than has been disclosed to this date.
According to the invention, there is provided a method for heating a subterranean formation having hydrocarbons, the method comprising: (a) providing at least a first conductor and a second conductor, wherein (i) the first and second conductors are spaced-apart in the formation, and (ii) there is electrical connectivity between the first and second conductors; (b) establishing at least a first electrode zone and a second electrode zone, each electrode zone having electrolyte, around the first and second conductors, respectively, and thereby creating a target region, having a center point, between opposing faces of the first and second electrode zones, wherein each electrode zone has an average effective radius that is at least about 2.3% of the distance between the centerline of the first conductor and the centerline of the second conductor; and (c) establishing at least about a 50% difference in electrical conductivity between the target region and independently each of the first and second electrode zones, wherein the electrical conductivity of the first and second electrode zones are each independently greater than an initial electrical conductivity of the target region, wherein the initial electrical conductivity of the target region is the average electrical conductivity, prior to applying an electric potential difference between the first and second electrode zones, in a substantially spherical portion centered around the center point of the target region, the substantially spherical portion of the target region having a radius of about 15% of the average spacing between opposing faces of the first and second electrode zones; so that when an electric potential difference is applied between the first and second electrode zones, a substantially diffuse distribution of increased temperature values is generated within the target region during at least the first 10% of a time interval when the electric potential difference is applied.
According to the invention, there is also provided a method for heating a subterranean formation having hydrocarbons, the method comprising: (a) providing at least a first conductor and a second conductor, wherein (i) the first and second conductor are spaced-apart in the formation, and (ii) there is electrical connectivity between the first and second conductors; (b) establishing at least a first electrode zone and a second electrode zone, each electrode zone having electrolyte, around the first and second conductors, respectively, and thereby creating a target region, having a center point, between opposing faces of the first and second electrode zones, wherein each electrode zone has an average effective radius that is at least about 2.3% of the distance between the centerline of the first conductor and the centerline of the second conductor; and (c) establishing at least about a 50% difference in electrical conductivity between the target region and independently each of the first and second electrode zones, wherein the electrical conductivity of the first and second electrode zones are each independently greater than an initial electrical conductivity of the target region, wherein the initial electrical conductivity of the target region is the average electrical conductivity, prior to applying an electric potential difference between the first and second electrode zones, in a substantially spherical portion centered around the center point of the target region, the substantially spherical portion of the target region having a radius of about 15% of the average spacing between opposing faces of the first and second electrode zones; so that at about 10% of a predetermined time interval over which an electric potential difference is continuously applied between the first and second electrode zones, there is at most about 60% deviation between the maximum and minimum values for a gamma ratio, xcex93, generated within the target region, wherein %xcex93 deviation is calculated as:
%xcex93Deviation=[ (xcex93maxxe2x88x92xcex93min)/xcex93max]xc3x97100
where
%xcex93 Deviation is the deviation of xcex93 values determined in a target region divided into n imaginary layers, wherein each imaginary layer has a highest temperature Tn at a point radially located a distance x from the first conductor and the thickness of the imaginary layer is determined by the length of an imaginary line parallel to and a radial distance x from the first conductor, wherein the temperature values along the imaginary line fall in a range Tnxe2x89xa7Txe2x89xa70.85Tn, as measured at about the initial 10% of the continuous electric heating time interval;
n is greater than or equal to 2;
xcex93max is the highest xcex93 of the n respective xcex93 values determined in the n layers at about the initial 10% of the continuous electric heating time interval;
xcex93min is the lowest xcex93 of the n respective xcex93 values determined in the n layers at about the initial 10% of the continuous electric heating time interval; and
xcex93 is a ratio of a rate of temperature increase for the portion of the target region having the highest temperature value versus a rate of temperature increase at an effective mid-point between the first and second electrode zones.
According to the invention, there is further provided a method for heating a subterranean formation having hydrocarbons, the method comprising: (a) providing at least a first conductor and a second conductor, wherein (i) the first and second conductor are spaced-apart in the formation, and (ii) there is electrical connectivity between the first and second conductors; (b) establishing at least a first electrode zone and a second electrode zone, each electrode zone having electrolyte, around the first and second conductors, respectively, and thereby creating a target region, having a center point, between opposing faces of the first and second electrode zones, wherein each electrode zone has an average effective radius that is at least about 2.3% of the distance between the centerline of the first conductor and the centerline of the second conductor; and (c) establishing at least about a 50% difference in electrical conductivity between the target region and independently each of the first and second electrode zones, wherein the electrical conductivity of the first and second electrode zones are each independently greater than the initial electrical conductivity of the target region, wherein the initial electrical conductivity of the target region is the average electrical conductivity, prior to applying an electric potential difference between the first and second electrode zones, in a substantially spherical portion centered around the center point of the target region, the substantially spherical portion of the target region having a radius of about 15% of the average spacing between the opposing faces of the first and second electrode zones; so that at about 10% of a predetermined time interval over which an electric potential difference is continuously applied between the first and second electrode zones, there is at most about 35% deviation between the highest and lowest maximum temperatures, Tmax, generated within the target region, wherein %Tmax deviation is calculated as:
%TmaxDeviation=[(Tmaxxe2x88x92highxe2x88x92Tmaxxe2x88x92low)/Tmaxxe2x88x92high]xc3x97100
where
%Tmax Deviation is the deviation of Tmax values determined in a target region divided into n imaginary layers, wherein each imaginary layer has a highest temperature Tn at a point radially located a distance x from the first conductor and the thickness of the imaginary layer is determined by the length of an imaginary line parallel to and a radial distance x from the first conductor, wherein the temperature values along the imaginary line fall in a range Tnxe2x89xa7Txe2x89xa70.85Tn, as measured at about the initial 10% of a continuous electric heating time interval;
n is greater than or equal to 2;
Tmaxxe2x88x92high is the highest Tmax of the n respective Tmax values determined in the n layers at about the initial 10% of the continuous electric heating time interval; and
Tmaxxe2x88x92low is the lowest Tmax of the n respective Tmax values determined in the n layers at about the initial 10% of the continuous electric heating time interval.